FAQ
TL;DR: VW Bosch alternators that look identical still vary by output (70A–140A) due to internal rotor/stator and rectifier differences. As the OP notes, “the difference is in the max charging current.” [Elektroda, tata1, post #16955559]
Why it matters: This FAQ helps VW owners and techs pick, swap, or diagnose Bosch alternators without guesswork.
- 70A, 90A, 120A, 140A are common VW Bosch ratings; part examples: 028 903 028C/D/E and 06F 903 023J. [Elektroda, tata1, post #16955559]
- Higher output uses larger rotor/stator and, between 120A and 140A, mainly stronger diode packs. [Elektroda, Strumien swiadomosci swia, post #16958818]
- “Winding wire thickness and amount” are key to current capability. [Elektroda, 762302, post #16955603]
- Rectifier design and winding leads can differ across units. [Elektroda, grzegorzbuczylo, post #16958702]
- Housings/mounts interchange; users swap 90A into a 140A slot temporarily. [Elektroda, tata1, post #16961167]
Quick Facts
- Rated outputs discussed: 70A, 90A, 120A, 140A; all VW-fit Bosch. [Elektroda, tata1, post #16955559]
- 120A and 140A share rotor size; 140A uses beefier diodes. [Elektroda, Strumien swiadomosci swia, post #16958818]
- More or thicker stator windings raise current capacity. [Elektroda, 762302, post #16955603]
- Rectifier topology and number of winding leads may vary. [Elektroda, grzegorzbuczylo, post #16958702]
- Visual cue: 140A looks slightly “inflated”; body protrudes past rear cover more than 90A. [Elektroda, tata1, post #16961254]
What actually determines the max charging current on these Bosch alternators?
Core dimensions and copper do. Larger rotor/stator iron and wider yoke allow more flux and copper, raising output. Rectifier and regulator choices then align to the higher allowable current and heat. “The higher the current, the larger the rotor, and hence the larger the stator.” [Elektroda, Strumien swiadomosci swia, post #16958818]
Are the 70A, 90A, 120A, and 140A units physically different or just the regulators?
They are physically different. Higher-amp versions use larger rotor/stator packs, and 140A strengthens the diode pack compared with 120A. Regulators are matched to the magnetizing current and switching demands, not the only difference. [Elektroda, Strumien swiadomosci swia, post #16958818]
Does increasing the number of phases automatically increase alternator current?
No. More phases alone do not raise the rated current. The excitation flux and heat removal limit current first; iron and cooling capacity dominate. Extra phases help ripple and packaging, not the ceiling. [Elektroda, robokop, post #16961234]
Can I swap a 90A alternator into a car that came with 140A?
Yes, as a temporary or light-duty solution. Mounting points match, and users report direct swaps to keep the car running. Expect less headroom for accessories under heavy loads. [Elektroda, tata1, post #16961167]
Will a 120A alternator overcharge a big AGM battery?
No, not if the regulator and wiring are healthy. One user runs a 120A alternator with a VARTA AGM 92Ah for a year without recharge issues. “There is no problem with recharging.” [Elektroda, grzegorzbuczylo, post #16958702]
Could a higher-output alternator ‘cook’ an even larger battery?
Within reason, yes—the system can support larger batteries when regulated correctly. A forum reply notes the 120A unit can handle an even larger battery without harm in normal use. [Elektroda, 767667, post #16958721]
What differences exist between 120A and 140A Bosch units?
They share rotor size, but the 140A version uses a stronger rectifier (diode pack). That change supports higher continuous current at temperature. [Elektroda, Strumien swiadomosci swia, post #16958818]
Is there a quick visual check to tell 140A from 90A?
Yes. Viewed from the pulley side toward the rear cover, the 140A body protrudes farther, giving a slightly “inflated” look versus 90A. [Elektroda, tata1, post #16961254]
Do regulators differ across these alternators?
Yes. Because magnetizing current and response differ, regulators vary in switching behavior and permissible current. Higher outputs require suitable regulator characteristics and thermal margins. [Elektroda, robokop, post #16958780]
Can small alternators maintain large-capacity batteries?
Yes, given realistic loads and drive time. One expert cites tractors with 55–75A alternators feeding 170Ah batteries without issues. That highlights usage patterns over raw capacity. [Elektroda, robokop, post #16958780]
What do ‘rotor’ and ‘stator’ mean here?
In this thread, the rotor is the excited magnet assembly; the stator is the winding yoke that sets output with wire size and width. Both scale with target current. [Elektroda, robokop, post #16958780]
How do winding choices impact output?
Thicker wire and more turns per phase reduce resistance and raise current capability, within the iron’s flux limits. Or as one reply states, “Winding wire thickness and amount.” [Elektroda, 762302, post #16955603]
Do rectifier diodes and winding leads vary between models?
Yes. Users report differences in diode count and the number of winding leads, aligning the rectifier to the alternator’s current rating. [Elektroda, grzegorzbuczylo, post #16958702]
Are Bosch regulator part numbers interchangeable across outputs?
The thread notes F00M145225 and F00M144128 used on 70A–120A, and F00M145296 on 140A, described as replacements by the OP. Verify on the unit label before purchase. [Elektroda, tata1, post #16955559]
What’s the main failure or edge case when pushing output?
Thermal limits. Even with clever phasing, excitation flux and heat dissipation cap safe current. Overheating can stress diodes and insulation under sustained high load. [Elektroda, robokop, post #16961234]
How can I compare two VW Bosch alternators quickly (90A vs 140A)?
- Check physical massing: higher output looks slightly larger around the stator window.
- Inspect rectifier: 140A uses a stronger diode pack than 120A.
- Note that rotor size tracks current rating on these families. [Elektroda, Strumien swiadomosci swia, post #16958818]
